The invention relates to a multi-branch frequency-hopping receiver used in base stations of a cellular radio system, for example.
FIG. 1 shows as a block diagram a method in which a plurality of receivers have been connected to a common antenna A in a multi-branch radio station. The study of the system is affected by the fact that it concerns a frequency-hopping system, in which case the components of the system cannot be very selective on a fixed frequency. This kind of receiver can also be referred to as being "multichannel". In addition to this meaning, the term "channel" may refer, depending on the context, to the nominal frequency of a radio link or a combination of a frequency and a time slot, for example. In order to avoid confusion, in the present application the term "channel" refers to the nominal frequency of a radio link or a combination of a frequency and a time slot, in which case parallel receivers are referred to as "receiver branches".
A broad band-pass filter BPF1 follows antenna A in the direction of propagation of the received signal. It passes through the whole frequency area of the system. The function of the band-pass filter BPF1 is to attenuate image frequencies and other non-desired components of the spectrum. The next stage is a low noise amplifier LNA which is followed by a distribution amplifier DA. A signal is conducted from the distribution amplifier DA to all the receivers of the station so that a second band-pass filter BPF2, a mixer MIX and an intermediate frequency stage IF are at the front end of each receiver. The number of receiver branches in FIGS. 1 to 5 is 5 as an example, but there may be more or less of them within reason.
The coupling of the distribution amplifier DA to the receivers is not possible simply by connecting the inputs of the filters BPF2 in parallel as because of frequency hopping, the filters BPF2 cannot be made selective enough. This connection in parallel would cause various adapting problems. Instead, a signal is conducted to the band-pass filters BPF2 via a chain of power dividers PD.
It is previously known that in receivers implemented without frequency hopping, the broadband band-pass filters BPF2 of FIG. 1 can be replaced by narrowband filters. In this case the inputs of the filters BPF2 can be connected together under certain conditions and they can be connected directly to a low-noise amplifier LNA without the distribution amplifier DA and the divider chain PD.
As the divider divides the power into two, its theoretical attenuation is 3 dB, but in practice, attenuation is about 3.5 dB. In the case of five receiver branches the attenuation of three branches is 7 dB and of two branches 10.5 dB.
A low-noise amplifier LNA and a distribution amplifier DA generate noise in the image frequencies of the mixer. Therefore it is necessary to have after them band-pass filters BPF2 whose function it is to attenuate the penetration/leakage of the local oscillators of the mixers from one receiver to another. Down-conversion takes place in high signal level mixers MIX which are controlled by frequency-hopping local oscillators which are not shown separately. After the mixers MIX, the desired signals are on a fixed frequency, in which case they can be processed further.
For example, many high requirements set for the distribution amplifier DA and inflexibility of connections are problems in the prior art solution described above. The distribution amplifier DA has to generate high output power (about 1 W in the GSM environment). It has to be low-noise and very linear to avoid intermodulation distortion.
The second problem in the prior art solution is the inflexibility of the connection caused by the divider chain. The divider chain PD is formed of power dividers which induce the required impedance matchings. If there are N receivers, N-1.sub.1 dividers are needed altogether and at least some of the signal paths have them in sequence log.sub.2 N rounded up to the next integral number. For example, 5 to 8 receivers require at most 3 successive dividers but the ninth receiver would then have a chain of four dividers.
The third problem with prior art receivers is provided by band-pass filters BPF2 which have to pass the whole desired frequency band but attenuate the many frequencies outside the band as much as possible. Therefore several resonators are needed there and they have to tuned, generally manually in the manufacturing stage. In addition, band-pass filters are bulky and have a high loss.